1. Field of the Invention
Disclosed is a process for producing isobutylene by the dehydration of tertiary butyl alcohol, and more particularly to a process for producing substantially pure isobutylene containing no or relatively small amounts of higher oligomer byproducts. In one particular embodiment, disclosed is a process for producing substantially pure isobutylene by dehydration of tertiary butyl alcohol in the presence of a Y-zeolite catalyst having a silica to alumina ratio of less than or equal to about ten.
2. Description of the Related Art
Isobutane may be reacted with oxygen to form a peroxidation reaction product containing tertiary butyl hydroperoxide and tertiary butyl alcohol, along with minor amounts of acetone, methanol, various esters and acids. One typical use of tertiary butyl hydroperoxide manufactured in this manner is to produce epoxides. For example, the process for the manufacture of substituted epoxides from alpha-olefins such as propylene is discussed in U.S. Pat. No. 3,351,635. In this process, an organic hydroperoxide such as tertiary butyl hydroperoxide may be reacted with an olefinically unsaturated compound such as propylene in the presence of a soluble molybdenum catalyst. Products of this reaction include propylene oxide and tertiary butyl alcohol.
In one conventional vapor phase process, tertiary butyl alcohol may be used to produce isobutylene by way of a vapor phase reaction at very high temperatures, i.e., from about 350 to about 450xc2x0 C. Such conventional vapor phase processes typically are accompanied by a large consumption of energy and require expensive heating equipment and other costly hardware.
In one conventional liquid phase process for the dehydration of tertiary butyl alcohol, azeotroping agents, such as benzene or xylene, are used to remove water from the liquid phase reaction product. However, azeotroping agents are typically expensive and may cause production of relatively heavy molecular weight products that must be purged from the process system. Examples of such conventional liquid phase products are described in, for example, U.S. Pat. Nos. 4,165,343 and 4,155,945.
Other conventional processes for the dehydration of tertiary butyl alcohol include those processes employing ion exchange resins. One example of such a process for the dehydration of X tertiary butyl alcohol is described in U.S. Pat. No. 3,256,250. This process employs sulfonated and nitrated styrene divinyl benzene resin which may be utilized to produce isobutylene. U.S. Pat. No. 3,510,538 describes a process in which tertiary butyl alcohol is continuously dehydrated over a cation exchange resin with water formed during the process being continuously removed to increase dehydration rates. However, cation exchange resins are expensive, and typically cannot be operated at temperatures greater than about 110xc2x0 C. to about 120xc2x0 C. Furthermore, to obtain useful rates of dehydration and relatively high conversion percentages, water must be continuously removed from the system.
Other conventional process for the dehydration of tertiary butyl alcohol are employed to produce higher molecular weight conversion products. For example, U.S. Pat. No 5,157,192 describes a process for the conversion of tertiary butyl alcohols to C-8 olefins over certain zeolite catalysts, such as certain beta-zeolites. In this process, tertiary butyl alcohol may be converted to 2,2,4-trimethylpentane(diisobutylene). A two-step partial oxidation-dimerization process to yield C-8 olefins is described.
Disclosed herein is a process for producing isobutylene from tertiary butyl alcohol in the presence of Y-zeolites. Using the disclosed process, tertiary butyl alcohol may be dehydrated at relatively low temperatures and pressures to produce isobutylene in substantially quantitative yield with little or substantially no diisobutylene formed. Surprisingly, these results may be achieved using zeolites having a silica to aluminum ratio of less than about 10, and in the presence of water in the process. Advantageously, the disclosed method obtains good conversion of tertiary butyl alcohol with relatively low silica to alumina ratio. Low silica to alumina ratio zeolite catalysts typically cost less and are more readily available than zeolite catalysts having higher silica to alumina ratios.
Using the disclosed process, substantially high yields of isobutylene may be produced in the substantial absence of dimer byproducts, such as diisobutylene. Such substantially high yield isobutylene products may be advantageously produced for use, for example, in plants where it is desirable to reduce isobutylene to isobutane and recycle it. For example, isobutane may be recycled to a peroxidation reactor to make more tert-butylhydroperoxide. The tert-butylhydroperoxide may be used (with propylene) in the presence of a molybdenum catalyst to produce more propylene oxide and tert-butylalcohol. The tert-butylalcohol may then be recycled to the above process. Other end uses for which substantially high purity isobutylene product of the disclosed process may be employed include, but are not limited to, for the production of substantially high purity isobutylene for sale to polymer markets, and where it is desirable to produce a mixture of isobutylene and isobutane for alkylation.
Whether a tertiary butyl alcohol-containing feed stream contains a relatively large amount of other components or is substantially pure tertiary butyl alcohol, relatively high conversion of tertiary butyl alcohol and relatively high selectivity to isobutylene may be advantageously achieved using the disclosed method. One or more other advantages of embodiments of the disclosed method include, but are not limited to, no need for large consumption of energy and/or expensive heating equipment as required with high temperature conventional processes, no need for expensive azeotroping agents or purge of relatively heavy molecular weight azeotrope products that must be purged from the process system, no requirement for continuous removal of water from the system, etc.
In one respect, disclosed is a method of producing isobutylene, including: contacting tertiary butyl alcohol with a Y-zeolite catalyst to produce isobutylene; wherein the Y-zeolite catalyst may have a silica to alumina ratio of less than about 10. The contacting may occur as part of a batch process (e.g., in a kettle or any other batch process reaction vessel suitable for producing isobutylene from tertiary butyl alcohol using Y-zeolite catalysts as described herein), or as part of a continuous flow process employing a feed stream that includes tertiary butyl alcohol (e.g., in any continuous flow process reaction vessel suitable for producing isobutylene from tertiary butyl alcohol using Y-zeolite catalysts as described herein). In one embodiment, water may also be present with tertiary butyl alcohol in a batch process reaction vessel or continuous flow feed stream. In another embodiment, a feed stream may include from about 70% to about 100% by weight tertiary butyl alcohol by total weight of the feed stream. In m another embodiment, the feed stream may include from about 70% to less than about 100% by weight tertiary butyl alcohol by total weight of the feed stream, and from greater than about 0% to about 30% by weight water by total weight of the feed stream. In another embodiment, conversion of tertiary butyl alcohol may be from about 80% to about 100%, and selectivity to isobutylene may be from about 85% to about 100%.
In another respect, disclosed is a method of producing isobutylene, including: contacting a feed stream including tertiary butyl alcohol and water with a Y-zeolite catalyst to produce isobutylene; wherein the Y-zeolite catalyst may have a silica to alumina ratio of less than or equal to about 10; wherein the contacting occurs within a reaction vessel at a temperature of greater than or equal to about 140xc2x0 C., a pressure of from about 50 psig to 1000 psig, and a liquid hourly space velocity (xe2x80x9cLHSVxe2x80x9d) of from about 0.1 g/mL catalyst/hour to about 20.0 g/mL catalyst/hour; wherein conversion of tertiary butyl alcohol may be from about 80% to about 100%, and wherein selectivity to isobutylene may be from about 85% to about 100%. In one embodiment, the Y-zeolite catalyst may have a silica to alumina ratio of from about 5 to about 10, and in another embodiment from about 5 to about 6. In another embodiment, the contacting may occur at a temperature of from about 140xc2x0 C. to about 250xc2x0 C., and in yet another embodiment from about 180xc2x0 C. to about 250xc2x0 C. In yet another embodiment, conversion of tertiary butyl alcohol may be from about 90% to about 100%. In yet another embodiment, selectivity to isobutylene may be from about 95% to about 100%. In yet another embodiment, the contacting may occur at a temperature of from about 200xc2x0 C. to about 250xc2x0 C. In yet another embodiment, conversion of tertiary butyl alcohol may be from about 95% to about 100% and selectivity to isobutylene may be from about 95% to about 100%.
In another respect, disclosed is a method of producing isobutylene, including: contacting a feed stream comprising tertiary butyl alcohol and water with a Y-zeolite catalyst to produce isobutylene; wherein the Y-zeolite catalyst has a silica to alumina ratio of from about 5 to about 6; wherein the contacting occurs within a reaction vessel at a temperature of from about 140xc2x0 C. to about 200xc2x0 C., a pressure of from about 240 psig to about 275 psig, and a LHSV of from about 1 g/mL catalyst/hour to about 5 g/mL catalyst/hour; and wherein conversion of tertiary butyl alcohol is from about 80% to about 100%, and wherein selectivity to isobutylene is from about 85% to about 100%.
Catalysts suitable for use in the disclosed process include Y-zeolites having a SiO2/Al2O3 molar ratio (xe2x80x9csilica to alumina ratioxe2x80x9d) of less than or equal to about 10, alternatively of less than or equal to about 5. In one embodiment, Y-zeolites having a silica to alumina ratio of from about 1 to about 10, alternatively from about 5 to about 10, and further alternatively from about 5 to about 6, may be employed. In separate respective and alternative embodiments, Y-zeolites having a silica to alumina ratio in a range of from about x to about y may be employed, where for each respective embodiment the value of x may be selected from the range of values of from 0.1 to 10 and a corresponding value of y may selected from the range of values of from 0.1 to 10, with the proviso that x not equal y for a given embodiment.
Suitable Y-zeolite catalysts include any Y-zeolite or mixture of Y-zeolites effective to catalyze the conversion of tertiary butyl alcohol to isobutylene, and that has or have silica to alumina ratios as described elsewhere herein. Examples of specific suitable Y-zeolites include, but are not limited to, xe2x80x9cZEOLIST CBV-300xe2x80x9d, xe2x80x9cZEOLIST CBV-400xe2x80x9d, xe2x80x9cZEOLIST CBV-500xe2x80x9d (available from the Zeolist Corporation); xe2x80x9cUOP LZY-84xe2x80x9d; xe2x80x9cUOP LZY-64 available from UOP, and Engelhard xe2x80x9cLY-ZEOLITExe2x80x9d from Engelhard Corporation.
Suitable feed stocks for use in the disclosed process include, but are not limited to, any process stream containing tertiary butyl alcohol. Such a feed stock stream may be substantially pure tertiary butyl alcohol, or may contain tertiary butyl alcohol mixed with other components. For example, in one embodiment, a feed stock may be the effluent from a tertiary butyl alcohol/propylene oxide production facility, such as described in U.S. Pat. Nos. 4,873,380, 4,845,251, 4,742,179, and 4,704,482, which are incorporated herein by reference. Such an effluent stream contains mainly tertiary butyl alcohol, but may also contain impurities such as acetone, methanol, various acids and esters, as well as water. In this regard, water is typically present in tertiary butyl alcohol effluent as it comes from a tertiary butyl alcohol/propylene oxide plant. Thus, it will be understood with benefit of this disclosure that types of tertiary butyl alcohol-containing feed streams that may be treated using the disclosed method may contain tertiary butyl alcohol in relatively small amounts up to substantially pure tertiary butyl alcohol.
In any case, the disclosed method maybe employed to achieve relatively high conversion of any tertiary butyl alcohol content present in the feed stream with high conversion rates to isobutylene relative to conventional processes. In one embodiment, tertiary butyl alcohol content in the feed stream may range from about 50% to about 100% by weight of feed, alternatively from about 50% to about 75% by weight of feed. In another embodiment, tertiary butyl alcohol content in the feed stream may range from about 75% to about 99.99%, alternatively from about 75% to about 90% by weight of feed. In yet another embodiment, tertiary butyl alcohol content in the feed stream may range from about 95% to about 99.99% by weight of feed. In yet another embodiment, tertiary butyl alcohol content in the feed stream may range from about 70% to about 100% by weight of feed. In one embodiment, amounts of water which may be present in such a tertiary butyl alcohol-containing feed stream may range from substantially no water up to about 30% of water by weight of feed, or alternatively, from about 5% to about 20% of water by weight of feed. In another embodiment, amounts of water which may be present in such a tertiary butyl alcohol-containing feed stream may range from about .01% to about 10% of water by weight of feed. It will be understood with benefit of this disclosure that each of the preceding concentration ranges for feed stream components are exemplary only, and amounts of each of the indicated components may fall outside these ranges. Furthermore, it will be understood that other components may be present as well.
Process conditions which may be employed in the practice of the disclosed method include any effective conditions (e.g., temperature, pressure, process flow rate relative to catalyst, etc.) at which tertiary butyl alcohol may be converted to isobutylene using one or more of the disclosed Y-zeolite catalysts. In this regard, the disclosed method may be practiced as a liquid or vapor phase reaction.
In one embodiment, relatively low pressures and/or temperatures may be employed to enhance tertiary butyl alcohol conversion and selectivity to isobutylene. For example, process temperature may range from about 50xc2x0 C. to about 250xc2x0 C., alternatively from about 120xc2x0 C. to about 220xc2x0 C., alternatively from about 140xc2x0 C. to about 190xc2x0 C., alternatively from about 140xc2x0 C. to about 180xc2x0 C., alternatively from about 160xc2x0 C. to about 220xc2x0 C, alternatively from about 160xc2x0 C. to about 200xc2x0 C., alternatively from about 160xc2x0 C. to about 190xc2x0 C., and further alternatively from about 160xc2x0 C. to about 180xc2x0 C. In another embodiment, process temperature may range from about 160xc2x0 C. to about 250xc2x0 C., alternatively from about 180xc2x0 C. to about 250xc2x0 C., alternatively from about 180xc2x0 C. to about 220xc2x0 C., and further alternatively from about 180xc2x0 C. to about 200xc2x0 C. In another embodiment, process pressure may range from about 50 psig to 1000 psig, alternatively from about 100 psig to about 300 psig, alternatively from about 200 psig to about 300 psig, alternatively from about 240 psig to about 280 psig, alternatively from about 240 psig to about 275 psig, alternatively from about 260 psig to about 280 psig, alternatively from about 260 psig to about 275 psig, and further alternatively from about 260 psig to about 270 psig. Flow rates of tertiary butyl alcohol-containing feed stream relative to the disclosed Y-zeolite catalyst may range from about 0.1 g/mL catalyst/hour to about 20 g/mL catalyst/hour, alternatively from about 1.0 g/mL catalyst/hour to about 10.0 g/mL catalyst/hour, and further alternatively from about 1.0 g/mL catalyst/hour to about 5.0 g/mL catalyst/hour. However, process conditions outside these ranges are also possible. Furthermore, various combinations of the above pressure ranges, temperature ranges and/or flow rate ranges are possible, as so desired.
In one embodiment of the disclosed method and Y-zeolite catalysts, conversion of tertiary butyl alcohol (i.e., moles of tertiary butyl alcohol converted during reaction expressed as a percentage of moles of tertiary butyl alcohol originally present) may be from about 50% to about 100%, alternatively from about 60% to about 100%, alternatively from about 70% to about 100%, alternatively from about 80% to about 100%, alternatively from about 90% to about 100%, and further alternatively from about 95% to about 100%. In another embodiment, selectivity to isobutylene (i.e., moles of isobutylene formed during reaction expressed as a percentage of moles of tertiary butyl alcohol converted during reaction) may be from about 70% to about 100%, alternatively from about 80% to about 100%, alternatively from about 85% to about 100%, alternatively from about 88% to about 100%, alternatively from about 90% to about 100%, and further alternatively from about 95% to about 100%. However, conversion values and/or selectivity values outside these ranges are also possible. Furthermore, various combinations of the above conversion values and selectivity values are possible using embodiments of the disclosed method and catalysts.
It will be understood with benefit of this disclosure that in the practice of the disclosed method virtually any reaction scheme and/or any type of reaction vessel suitable for contacting tertiary butyl alcohol with the disclosed Y-zeolite catalyst may be employed, including batch and continuous flow methods. In one continuous reaction scheme embodiment, a tubular reaction vessel containing a fixed bed of the disclosed Y-zeolite catalyst may be employed, and operated in a single-pass mode or with one or more effluent recycle streams. Furthermore, one or more reaction vessels may be employed, with multiple reaction vessels operated in series and/or parallel. Continuous flow reaction vessels may be configured with any suitable flow direction, e.g., upflow, downflow, etc.
It will be understood by benefit of this disclosure that a continuous process scheme may be employed without a recycle stream using fractionation equipment, or may be employed with an unreacted tertiary butyl alcohol recycle stream in the absence of such fractionation equipment. When an unreacted tertiary butyl alcohol recycle stream is employed, water content may be increased in the reaction vessel.